Electrical discharge machining (EDM) is a popular electrode machining process. Electrodes for EDM are usually machined from a conductive material such as copper or graphite. Since the shape of the electrode, or at least the working portion of the electrode must correspond to the geometry of the desired, finished part, the electrode manufacturing process can be time-consuming depending upon the required complexity of the part shape. That is, the formation of the EDM electrode increases the cost and turnaround time for creating complex parts such as gear molds. Such complexity is particularly troublesome in the area of involute gear formation.
Further, during electrical discharge machining, the electrodes are subject to wear. Therefore, electrode life and cost are significant factors in the EDM process.
Even with the advent of computer generated design, measuring and feedback techniques to assist in effecting electrode design, the electrode machining still requires time, skill and considerable care. Advanced electrode systems often require several iterations.
Electric discharge machining removes material at a slow rate to provide the accuracy required in producing high precision parts. Mechanical cutters produce large chips, debris, and sharp edges on the structural member being cut and apply high loads of torque to the structural member during the cutting process. In contrast, EDM produces very small round chips, smooth edges and applies no torque to the structural member being cut.
In the EDM process, an electrode is brought in close proximity to an electrically conducting work piece creating a gap between the work piece and the electrode. The gap is gently flushed with a dielectric fluid as a pulse DC voltage is applied across the gap. The dielectric fluid is ionized at a localized spot as a large current flows across the gap, vaporizing a portion of the work piece. Thus, to make an EDM electrode for creating a mold having a particular gear form, (i.e. tooth shape and orientation), the electrode must include and fully define the gear form.
Heretofore, the gear forming electrode for an EDM process has been cut by generation. In general, in a gear generating machine, the generating tool can be considered as one of the gears in a conjugate pair with the EDEM electrode as the other gear. This generation is generally referred to as a "hobbing process". A hob is used to physically cut the involute surface on the gear teeth in the EDM electrode. However, hobbing is a time-consuming and expensive process. As industrial requirements force the total time from design to production to decrease, a significant need exists for the rapid generation of finished electrodes and molds.
In particular, this problem is evident in the injection molding of plastic gears. In such injection molding processes, the plastic is injected into a mold cavity, at least partially set and removed from the cavity, at which time the cavity is refilled.
Thus, for the formation of injection molded involute plastic gears, a blank EDM electrode is formed. The EDM electrode blank is then subjected to a hobbing process to form the involute curves of the desired gear in the EDM electrode. The EDM electrode is then used to form a corresponding involute gear form cavity in the mold blank.
Therefore, the need exists for a method of manufacturing an injection mold for the creation of plastic gears without requiring the expensive and time consuming hobbing process for forming an EDM electrode having an involute curve. The need also exists for forming EDM electrodes having involute surfaces without requiring the hobbing process.